There is much debate about this topic. The surgeon has several options available. The patella can be left unchanged while the rest of the joint is replaced. The patella can be resurfaced. This means the back of the kneecap is smoothed. It may be left that way to heal or a smooth, polypropylene (plastic) liner may be attached to the back of the patella.
Knee pain is the main problem with leaving the patella unchanged. Second to that, the arthritis may continue to get worse in the patella. The cartilage along the back of the patella that comes in contact with the rest of the knee joint can get worn unevenly, form bone spurs, or thicken in an attempt to protect the joint. When this happens, the patient may report "tightness" and loss of motion in the joint.
The surgeon will make a judgment call during the operation. Which option to choose is based on the present condition of the patella, your father's age, and his activity level.
One of the most important considerations in leaving the patient's patella intact is: how long will the cartilage surface hold up moving against the metal implant. Early studies show that leaving the patella unchanged may not have the best results.